Aluminum alloys are highly desirable for vehicle frame construction because they offer low density, good strength and corrosion resistance. Moreover, aluminum alloys can be employed to improve vehicle frame stiffness and performance characteristics. Moreover, it is believed that an aluminum vehicle frame retains the strength and crashworthiness that is typically associated with much heavier, conventional steel frame vehicle designs.
An important consideration for aluminum automotive body structures includes crashworthiness in conjunction with reducing the overall vehicle weight and/or improving vehicle performance. For the automotive applications, crashworthiness reflects the ability of a vehicle to sustain some amount of collision impact without incurring unacceptable distortion of the passenger compartment or undue deceleration of the occupants. Upon impact, the structure should deform in a prescribed manner; the energy of deformation absorbed by the structure should balance the kinetic energy of impact; the integrity of the passenger compartment should be maintained; and the primary structure should crush in such a manner as to minimize the occupant deceleration.
The demand for higher crash performance of automotive aluminum cast components has greatly increased, particularly with respect to body structures, including but not limited to: brackets, nodes (e.g. A-Post, B-Post, C-Post, etc.), crashboxes, crossmembers, subframes, and engine cradles, etc. However, the most common aluminum cast alloy, A356 has poor crushability even in T6 temper. One characterization of crashworthiness is critical fracture strain (CFS) as developed by Yeh. See J. R. Yeh, “The Development of an Aluminum Failure Model for Crashworthiness Design”, Report No. 99-016, 1999-03-11. The critical fracture strain (CFS) or A356-T6 is approximately 5-6%. Typically, the critical fracture strain (CFS) required for a crash sensitive components and applications is on the order of 10% or greater.
Therefore, a new alloy and heat treatment are needed for producing cast components with a balanced strength and crashworthiness.